Thin-walled, extensible articles such as gloves and condoms have long been made from natural rubber latex, which comprises an emulsion of natural rubber and water, stabilizers and vulcanizing agents. Typically, a form of an appropriate shape (optionally pre-coated with a coagulating solution) is dipped into the natural rubber latex mixture once or several times to build up a layer of the desired thickness. The water is then evaporated to leave behind a solid natural rubber latex film. The film is further vulcanized to provide adequate mechanical and physical properties.
Natural rubber latex offers numerous advantages for these articles, such as high durability, elasticity and good “tactility” or feeling to the user. In particular, the good tactility is believed to be a result of the low deformation stress of natural rubber latex at 10-500 percent elongations, and the high elastic recovery from these elongations.
However, natural rubber latex is not hypoallergenic due to the presence of residual surfactants, vulcanizing agents, stabilizing agents, antioxidants, or protein materials. Subsequently, individuals who are particularly susceptible to irritation or sensitization may experience allergic reactions from contacting natural rubber latex. Natural rubber latex films can also be weakened by their exposure to oil-based materials such as mineral oil, motor oil, etc.
In order to overcome the above disadvantages, various types of synthetic elastomeric polymer products have been developed instead. These articles are typically produced by dip forming from either water-based polymer, or solvent-based polymer systems that are made from the dissolution of synthetic rubber compositions in solvents.
Recently, a variety of methods and procedures have been described in the prior art for preparing thin and extensible particles with synthetic polymers. For example, U.S. Pat. No. 4,880,878 to Himes et al. describes a thermoplastic blend that absorbs less than 40% of an oil, and has a superior tensile strength. The blend comprises about 80 to about 20 phr of an elastomeric block copolymer that has a general configuration of A-B-A, wherein the B block is a butadiene hydrocarbon block that is consisted of about 35 to about 55 mole percent of condensed butadiene units in a 1,2 configuration, and about 20 to about 80 phr of a block copolymer that has a general configuration of A-B-A, wherein the B block is a hydrogenated butadiene hydrocarbon block consisting of about 18 to about 34 mole percent of condensed butadiene units in a 1,2 configuration.
Another example is U.S. Pat. Nos. 5,112,900 and 5,407,715 to Buddenhagen et al., which disclose an elastomeric liquid solution for producing gloves or condoms, comprising essentially of a block copolymer component that is comprised of at least two SEBS (styrene-block-ethylene-co-butylene-block-styrene) triblock copolymers that have different solution viscosity/copolymer concentration values; a plasticizer in an amount sufficient to provide tactility in dip formed products made from the composition; and a solvent in an amount sufficient to form a stable solution of the block copolymer component and the plasticizer. Preferably, three SEBS block copolymers form the block copolymer component, the plasticizer is mineral oil, and the plasticizer is present in an amount sufficient to reduce the deformation stress of the solid formed elastomer to less than about 5.5 MPa (millions of Pascals) at 500 percent elongation.
Another example is U.S. Pat. No. 6,639,007 to Plamthottam, which describes an elastomeric composition that includes a single SEBS block copolymer having at least about 15 weight percent of styrene end blocks, wherein the weight average molecular weight of the styrene end blocks is at least about 7,000 Daltons and the weight average molecular weight of ethylene-co-butylene midblocks is at least about 60,000 Daltons, and a plasticizer in an amount sufficient to provide tactility in products made from the composition. A form having the shape of the desired product is dipped into the composition to build up a film of the elastomeric composition on the form. The dip-formed elastomeric films are free of pinholes and resistant to oxidative and ozone attack. The films are particularly suitable for use in products such as examination surgical and industrial gloves and condoms.
However, these known synthetic rubber compositions do not have the required combination of strength, tactility and/or resistance to environmental damage required for many products such as examination and surgical gloves and condoms.
Therefore there remains a need for an improved elastomeric material for producing thin, dip-formed articles such as examination or surgical gloves and condoms. Such a material should have not only the required properties of strength and elastic elongation, but also be pinhole free when the article is formed and used, resistant to immediate environmental or damage occurring during storage or use, and be hypoallergenic. The present invention fulfills this need, and further provides related advantages.